2000 Ls v8 misfire

I recently had my motor rebuilt in my 2000 ls v8 (a different story). All was well for about a week then it started to have a random misfire. By random I mean it seems to run misfire free for 20 mins. in the morning when I first start it. After that it seems to get worse as it warms up. The misfire then seems to come and go.

I have had the computer read several times and it shows misfire on cylinder 3. I swapped around coils from 1 to 3 and still showed misfire on 3. These are still the original coils and unless I anyone has alternative ideas I will replace 1 thru 4 with new ones.

I am suspect of the computer and have been told it is possible that it's not sending proper firing info to cylinder and may need replaced.

I don't want to start replacing parts blindly. Is there a proper way to diagnose misfire w/o going broke?

Thanks in advance for any help.

Mike

Happens with an increase in heat, huh? Moving coils makes no difference, huh?

I would replace your plugs first thing. My money is on a hairline crack in an insulator that expands enough under heat to cause an intermittent misfire.

I have a similar problem as above, but no codes or CEL. Is there a way to determine which cylinder is my problem?

If you can find a scan tool with the I/M Readiness test, the readiness test should display which cylinder(s) appears to be misfiring and how many misfire instances were counted. If the count is not above the set threshhold (something like 1 misfire per 200 revolutions) it will report a P030x code where "x" is the cylinder number. The misfiring cylinder will be determined based on the knock sensor and Cylinder IDentification (CID) sensor position, or Camshaft Position (CMP) Sensor.

If you can also monitor the exhaust air/fuel ratio via the oxygen sensor voltage and notice a rich mixture during the stumble/misfire, then your issue is probably a bad plug, low compression, or most likely, a bad coil on that cylinder. Once you find the suspect cylinder, remove the coil pack and inspect it for evidence of oil. Leaky valve cover gaskets are pretty common to fail.

-Rod